1. Field of the Invention
The present invention relates to optical disc players and more particularly to optical disc players designed to read optical discs that are at least in part unmetallized.
2. Background of the Invention
FIG. 1 shows a typical optical disc 10, in this case a compact disc. In order to read this disc, light 20 is directed toward the disc and strikes from the bottom, passing through a transparent plastic layer 30. The light is generated by a semiconductor laser, e.g., a 780 nanometer aluminum, gallium-arsenide semiconductor laser. Typically, the transparent plastic layer is polycarbonate as it has the best characteristics for such use, i.e., the combination of optical characteristics, heat resistance, moisture resistance, formability, etc. A pit 40 which corresponds to the signal is shown as a depression or projection in the transparent plastic layer 30. An extremely thin metal layer 50, i.e., the metallization, follows the contour of the pits 40. The metal layer 50 is typically 0.1 micron thick and formed via sputtering techniques. The purpose of the metal layer 50 is solely to increase the reflectivity of the disc. Accordingly, aluminum is typically used for the metal layer 50 due to its high reflectivity. For a typical metallized compact disc, i.e., a compact disc including metal layer 50, the value of reflectivity for light 20 is in the neighborhood of 80 percent at the metallized aluminum--pit surface. In contrast, with an unmetallized, i.e., no metal layer 50, disc reflectivity has been shown to be in the neighborhood of 2-4 percent, i.e., light which reflects back from the pit plane.
A hard resin layer 60 is provided above the metal layer 50 to protect the signal layer of the disc. A label or other design can be silk-screened over this layer 60.
The semiconductor laser of a conventional compact disc player is included in an optical pickup. The optical pickup detects (reads) the optical signal and can be thought of as the heart of a compact disc system. In addition to the semiconductor laser, an optical pickup generally includes, among other things, photodetectors and a series of lenses. The photodetectors in turn include light sensing elements and a pre-amplification section. In typical compact disc players that employ metallized discs, the power of the beam exiting the optical pickup, or more specifically, exiting the semiconductor laser to read the disc, is in the range of 0.3 to 0.5 milliwatt incident power. The average power of the reflected light would thus be 80% of this 0.3 to 0.5 milliwatt incident power. While various servos exist to control the optical pickup, e.g., a focus servo and a tracking servo, there is no control system presently being employed in optical disc readers such as compact disc players for controlling the average power level of the reflected beam. However, such a servo may not be particularly necessary with present day systems employing uniformly metallized discs in good repair and substantially constant output semiconductor lasers having focus servos, as the 80 percent reflectivity at the metallized aluminum--pit surface results in a power level of the reflected light being sufficient to be discerned by the photodetectors of the optical pickup.
In certain applications it would be desirable to read a compact disc without a metallized layer 50. For example, as known, the production of compact discs is a multi-phase process, and each phase is subject to various problems requiring appropriate quality control. The quality controls include: glass master testing, stamper testing, metallized disc testing and finished product testing. A method of analyzing compact discs prior to the process of metallization would be a useful aid in trouble shooting the manufacturing process. For example, by determining specific parameters before and after metal is applied, a determination as to the effects of metallization could be made. However, while it is advantageous to find problems as early as possible in the manufacturing process to minimize process time, production costs and excess rejected product, in conventional compact disc manufacturing processes it is not until the step of metallized disc testing that it can be determined whether information has been properly recorded. The reason for this is because conventional compact disc players having semiconductor lasers in the range of 0.3 to 0.5 milliwatt provide insufficient power to produce a reflected light signal strong enough to be read by typical optical pickups due to the poor reflectivity of the plastic (plastic layer 30)/air (hard resin layer 60 has not yet been formed) interface (approximately 4 percent). Accordingly, there is a need for the ability to fully play and analyze a clear compact disc to provide feedback on quality immediately after molding.
Another application for an unmetallized disc would be for storing long term archival data in a format such as CD-ROM, or for long term storage of other audio or video formatted CD information. The reason an unmetallized disc would be preferable to a metallized disc is that the metallization is the only part of a compact disc possibly subject to the very long term effects of aging and environmental conditions. By storing information on a clear disc, the permanency of the record would be assured. Thus, a player for reading unmetallized compact discs would be desirable.
Finally, in certain instances it would be desirable to play a compact disc that is partially metallized and partially unmetallized. An example of such an application would be a compact disc that is damaged, i.e., part of the metal is inadvertently removed or damaged. Furthermore, it has been also claimed that severe conditions such as those found in mechanized search and rescue vehicles or other applications involving extreme environmental conditions or simply very long time periods may cause premature deterioration of part or all of the metallized layer thus rendering a disc unreadable by conventional compact disc players or CD-ROM players. In this case, the problems presented by conventional players are two-fold. First, not only do present compact disc players provide insufficient power to read the unmetallized portions of the disc, but because no means are provided for adjusting laser power in response to changes in average power of the reflected beam resulting from moving from metallized to unmetallized portions of the disc (or vice versa), such changes cannot be compensated for. Specifically, there is no fast, i.e., less than the 1.9 milliseconds of lost symbols which can be corrected by the cross interleave Reed-Solomon code ("CIRC") error correction system of typical CD players, power control servo currently employed to adjust for instantaneous changing disc conditions. Accordingly, it would be advantageous to be able to read a compact disc which is at least in part unmetallized.